In the past, people met face-to-face when they wanted to communicate. But, in today's mobile, widely dispersed, and increasingly interconnected society, people often need to communicate with others who are far away. In order to facilitate this communication, teleconferencing and video conferencing are gaining in popularity. In teleconferencing, both parties have a conferencing system that may include a microphone and a speaker, and the parties are connected to each other via a network, so that they can converse. In video conferencing, the parties also have a camera and a video monitor, so the parties can converse while viewing still or moving video images of each other.
Teleconferencing and video conferencing systems suffer from the problem of acoustic echo, which is a delayed and distorted version of an original sound reflected back to its source. A traditional system typically includes a speaker/microphone pair on both ends (called the near-end and the far-end) of the connection. When near-end participants talk, their voices are picked up by the near-end microphone, transmitted to the far-end, and presented via the far-end speaker. The far-end microphone will also pick up this signal, directly or indirectly, and the far-end system will send it back to the near-end. This causes the near-end participants to hear a delayed and distorted version of their own speech, which is annoying.
Previous systems attempted to suppress echo by suppressing the signal from the microphone at one end when audio from the other end is present. Unfortunately, this leads to clipping of the voice signal and reduced intelligibility. More sophisticated systems employ active noise cancellation using a filter adapted to model the characteristics of the feedback paths between the speaker and the microphone. This suffers from the problems of high computational load of the filtering operation, difficulty in selecting an appropriate gain for the filter, and divergence of the filter when participants at the near and far ends are speaking simultaneously.
Although the problem of echo has been described in the context of teleconferencing and video conferencing, it can also occur when placing a telephone call with a speakerphone or whenever a speaker produces sound that enters a microphone.